A person who is just "there." Nobody really knows why, other than they show up and don't say a word. Possible reasons for this could be:
-They don't know what to say
-They are pretending to have friends
-They are trying to scare you
-They are there just to be seen with you, as if they have some kind of relationship with you.

A guy who hangs around your girlfriend as a "friend" who is really just waiting for the day when you and your significant other are on the rocks. That is when he swoops in to make a move, as the comforter, supporter, and basher of her current bf. Women are oblivious to them entirely.